yea i just found that out... this is gonna be a lot of work haha
It doesn't have to be alot of work. If you already have a sealed tunnel that you made displacements from do this:
Select all displacements in tunnel. Shift-drag over a unit (copy it), drag it back, hit displacement button and destroy them all at once.
Hide displacements, select all those brushes again, apply no draw.
unhide displacements. You only need to clean up stuff that clips real bad, enough to mess up pathfinding.
For example: dustbowls first spawn is all displacements. There are ALOT of spots where the nodraw brushes stick through them. Really all it does is 'player clip' holes that go into the wall far. So you can actually smooth out the gameplay area a bit like this without player clip.
nodraw wont cast shadows on the displacements or be seen in anyway.
(stickies might float a little, or a weird corner sticking out could block a player, just clean those things up some)